Читать книгу Water Supply: the Present Practice of Sinking and Boring Wells - Ernest Spon - Страница 4

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Nearly every civil engineer is familiar with the fact that certain porous soils, such as sand or gravel, absorb water with rapidity, and that the ground composed of them soon dries up after showers. If a well be sunk in such soils, we often penetrate to considerable depths before we meet with water; but this is usually found on our approaching some lower part of the porous formation where it rests on an impervious bed; for here the water, unable to make its way downwards in a direct line, accumulates as in a reservoir, and is ready to ooze out into any opening which may be made, in the same manner as we see the salt water filtrate into and fill any hollow which we dig in the sands of the shore at low tide. A spring, then, is the lowest point or lip of an underground reservoir of water in the stratification. A well, therefore, sunk in such strata will most probably furnish, besides the volume of the spring, an additional supply of water.

The transmission of water through a porous medium being so rapid, we may easily understand why springs are thrown out on the side of a hill, where the upper set of strata consist of chalk, sand, and other permeable substances, whilst those lying beneath are composed of clay or other retentive soils. The only difficulty, indeed, is to explain why the water does not ooze out everywhere along the line of junction of the two formations, so as to form one continuous land-soak, instead of a few springs only, and these oftentimes far distant from each other. The principal cause of such a concentration of the waters at a few points is, first, the existence of inequalities in the upper surface of the impermeable stratum, which lead the water, as valleys do on the external surface of a country, into certain low levels and channels; and secondly, the frequency of rents and fissures, which act as natural drains. That the generality of springs owe their supply to the atmosphere is evident from this, that they vary in the different seasons of the year, becoming languid or entirely ceasing to flow after long droughts, and being again replenished after a continuance of rain. Many of them are probably indebted for the constancy and uniformity of their volume to the great extent of the subterranean reservoirs with which they communicate, and the time required for these to empty themselves by percolation. Such a gradual and regulated discharge is exhibited, though in a less perfect degree, in all great lakes, for these are not sensibly affected in their levels by a sudden shower, but are only slightly raised, and their channels of efflux, instead of being swollen suddenly like the bed of a torrent, carry off the surplus water gradually.

An Artesian well, so called from the province of Artois, in France, is a shaft sunk or bored through impermeable strata, until a water-bearing stratum is tapped, when the water is forced upwards by the hydrostatic pressure due to the superior level at which the rain-water was received.

Among the causes of the failure of Artesian wells, we may mention those numerous rents and faults which abound in some rocks, and the deep ravines and valleys by which many countries are traversed; for when these natural lines of drainage exist, there remains only a small quantity of water to escape by artificial issues. We are also liable to be baffled by the great thickness either of porous or impervious strata, or by the dip of the beds, which may carry off the waters from adjoining high lands to some trough in an opposite direction,—as when the borings are made at the foot of an escarpment where the strata incline inwards, or in a direction opposite to the face of the cliffs.

Fig. 1.

Fig. 2.

As instances of the way in which the character of the strata may influence the water-bearing capacity of any given locality, we give the following examples, taken from Baldwin Latham’s papers on ‘The Supply of Water to Towns.’ Fig. 1 illustrates the causes which sometimes conduce to a limited supply of water in Artesian wells. Rain descending on the outcrop E F of the porous stratum A, which lies between the impervious stratum B B, will make its appearance in the form of a spring at S; but such spring will not yield any great quantity of water, as the area E F, which receives the rainfall, is limited in its extent. A well sunk at W, in a stratum of the above description, would not be likely to furnish a large supply of water, if any. The effect of a fault is shown in Fig. 2. A spring will in all probability make its appearance at the point S, and give large quantities of water, as the whole body of water flowing through the porous strata A is intercepted by being thrown against the impermeable stratum B. Permeable rock intersected by a dyke and overlying an impermeable stratum is seen in Fig. 3. The water flowing through A, if intersected by a dyke D, will appear at S in the form of a spring, and if the area of A is of large extent, then the spring S will be very copious. As to the depth necessary to bore certain wells, in a case similar to Fig. 4, owing to the fault, a well sunk at A would require to be sunk deeper than the well B, although both wells derive their supply from the same description of strata. If there is any inclination in the water-bearing strata, or if there is a current of water only in one direction, then one of the wells would prove a failure owing to the proximity of the fault, while the other would furnish an abundant supply of water.

Fig. 3.

It should be borne in mind that there are two primary geological conditions upon which the quantity of water that may be supplied to the water-bearing strata depends; they are, the extent of superficial area presented by these deposits, by which the quantity of rain-water received on their surface in any given time is determined; and the character and thickness of the strata, as by this the proportion of water that can be absorbed, and the quantity which the whole volume of the permeable strata can transmit, is regulated. The operation of these general principles will constantly vary in accordance with local phenomena, all of which must, in each separate case, be taken into consideration.

Fig. 4.

The mere distance of hills or mountains need not discourage us from making trials; for the waters which fall on these higher lands readily penetrate to great depths through highly-inclined or vertical strata, or through the fissures of shattered rocks; and after flowing for a great distance, must often reascend and be brought up again by other fissures, so as to approach the surface in the lower country. Here they may be concealed beneath a covering of undisturbed horizontal beds, which it may be necessary to pierce in order to reach them. The course of water flowing underground is not strictly analogous to that of rivers on the surface, there being, in the one case, a constant descent from a higher to a lower level from the source of the stream to the sea; whereas, in the other, the water may at one time sink far below the level of the ocean, and afterwards rise again high above it.

For the purposes under consideration, we may range the various strata of which the outer crust of the earth is composed under four heads, namely: 1, drift; 2, alluvion; 3, the tertiary and secondary beds, composed of loose, arenaceous and permeable strata, impervious, argillaceous and marly strata, and thick strata of compact rock, more or less broken up by fissures, as the Norwich red and coralline crag, the Molasse sandstones, the Bagshot sands, the London clay, and the Woolwich beds, in the tertiary division; and the chalk, chalk marl, gault, the greensands, the Wealden clay, and the Hastings sand; the oolites, the has, the Rhætic beds, and Keuper, and the new red sandstone, in the secondary division; and 4, the primary beds, as the magnesian limestone, the lower red sand, and the coal measures, which consist mainly of alternating beds of sandstones and shales with coal.

The first of these divisions, the drift, consisting mainly of sand and gravel, having been formed by the action of flowing water, is very irregular in thickness, and exists frequently in detached masses. This irregularity is due to the inequalities of the surface at the period when the drift was brought down. Hollows then existing would often be filled up, while either none was deposited on level surfaces, or, if deposited, was subsequently removed by denudation. Hence we cannot infer when boring through deposits of this character that the same, or nearly the same, thickness will be found at even a few yards’ distance. In valleys this deposit may exist to a great depth, the slopes of hills are frequently covered with drift, which has either been arrested by the elevated surface or brought down from the upper portions of that surface by the action of rain. In the former case the deposits will probably consist of gravel, and in the latter, of the same elements as the hill itself.

The permeability of such beds will, of course, depend wholly upon the nature of the deposit. Some rocks produce deposits through which water percolates readily, while others allow a passage only through such fissures as may exist. Sand and gravel constitute an extremely absorbent medium, while an argillaceous deposit may be wholly impervious. In mountainous districts springs may often be found in the drift; their existence in such formations will, however, depend upon the position and character of the rock strata; thus, if the drift cover an elevated and extensive slope of a nature similar to that of the rocks by which it is formed, springs due to infiltration through this covering will certainly exist near the foot of the slope. Upon the opposite slope, the small spaces which exist between the different beds of rock receive these infiltrations directly, and serve to completely drain the deposit which, in the former case, is, on the contrary, saturated with water. If, however, the foliations or the joints of the rocks afford no issue to the water, whether such a circumstance be due to the character of their formation, or to the stopping up of the issues by the drift itself, these results will not be produced.

It will be obvious how, in this way, by passing under a mass of drift the water descending from the top of hill slopes reappears at their foot in the form of springs. If now we suppose these issues stopped, or covered by an impervious stratum of great thickness, and this stratum pierced by a boring, the water will ascend through this new outlet to a level above that of its original issue, in virtue of the head of water measured from the points at which the infiltration takes place to the point in which it is struck by the boring.

Alluvion, like drift, consists of fragments of various strata carried away and deposited by flowing water; it differs from the latter only in being more extensive and regular, and, generally, in being composed of elements brought from a great distance, and having no analogy with the strata with which it is in contact. Usually it consists of sand, gravel, rolled pebbles, marls or clays. The older deposits often occupy very elevated districts, which they overlie throughout a large extent of surface. At the period when the large rivers were formed, the valleys were filled up with alluvial deposits, which at the present day are covered by vegetable soil, and a rich growth of plants, through which the water percolates more slowly than formerly. The permeability of these deposits allows the water to flow away subterraneously to a great distance from the points at which it enters. Springs are common in the alluvion, and more frequently than in the case of drift, they can be found by boring. As the surface, which is covered by the deposit, is extensive, the water circulates from a distance through permeable strata often overlaid by others that are impervious. If at a considerable distance from the points of infiltration, and at a lower level, a boring be put down, the water will ascend in the bore-hole in virtue of its tendency to place itself in equilibrium. Where the country is open and uninhabited, the water from shallow wells sunk in alluvion is generally found to be good enough and in sufficient quantity for domestic purposes.

The strata of the tertiary and secondary beds, especially the latter, are far more extensive than the preceding, and yield much larger quantities of water. The chalk is the great water-bearing stratum for the larger portion of the south of England. The water in it can be obtained either by means of ordinary shafts, or by Artesian wells bored sometimes to great depths, from which the water will frequently rise to the surface. It should be observed that water does not circulate through the chalk by general permeation of the mass, but through fissures. A rule given by some for the level at which water may be found in this stratum is, “Take the level of the highest source of supply, and that of the lowest to be found. The mean level will be the depth at which water will be found at any intermediate point, after allowing an inclination of at least 10 feet a mile.” This rule will also apply to the greensand. This formation contains large quantities of water, which is more evenly distributed than in the chalk. The gault clay is interposed between the upper and the lower greensand, the latter of which also furnishes good supplies. In boring into the upper greensand, caution should be observed so as not to pierce the gault clay, because water which permeates through that system becomes either ferruginous, or contaminated by salts and other impurities.

The next strata in which water is found are the upper and inferior oolites, between which are the Kimmeridge and Oxford clays, which are separated by the coral rag. There are instances in which the Oxford clay is met with immediately below the Kimmeridge, rendering any attempt at boring useless, because the water in the Oxford clay is generally so impure as to be unfit for use. And with regard to finding water in the oolitic limestone, it is impossible to determine with any amount of precision the depth at which it may be reached, owing to the numerous faults which occur in the formation. It will therefore be necessary to employ the greatest care before proceeding with any borings. Lower down in the order are the upper has, the marlstone, the lower has, and the new red sandstone. In the marlstone, between the upper and lower beds of the has, there may be found a large supply of water, but the level of this is as a rule too low to rise to the surface through a boring. It will be necessary to sink shafts in the ordinary way to reach it. In the new red sandstone, also, to find the water, borings must be made to a considerable depth, but when this formation exists a copious supply may be confidently anticipated, and when found the water is of excellent quality.

Every permeable stratum may yield water, and its ability to do this, and the quantity it can yield, depend upon its position and extent. When underlaid by an impervious stratum, it constitutes a reservoir of water from which a supply may be drawn by means of a sinking or a bore-hole. If the permeable stratum be also overlaid by an impervious stratum, the water will be under pressure and will ascend the bore-hole to a height that will depend on the height of the points of infiltration above the bottom of the bore-hole. The quantity to be obtained in such a case as we have already pointed out, will depend upon the extent of surface possessed by the outcrop of the permeable stratum. In searching for water under such conditions a careful examination of the geological features of the district must be made. Frequently an extended view of the surface of the district, such as may be obtained from an eminence, and a consideration of the particular configuration of that surface, will be sufficient to enable the practical eye to discover the various routes which are followed by the subterranean water, and to predicate with some degree of certainty that at a given point water will be found in abundance, or that no water at all exists at that point. To do this, it is sufficient to note the dip and the surfaces of the strata which are exposed to the rains. When these strata are nearly horizontal, water can penetrate them only through their fissures or pores; when, on the contrary, they lie at right-angles, they absorb the larger portion of the water that falls upon their outcrop. When such strata are intercepted by valleys, numerous springs will exist. But if, instead of being intercepted, the strata rise around a common point, they form a kind of irregular basin, in the centre of which the water will accumulate. In this case the surface springs will be less numerous than when the strata are broken. But it is possible to obtain water under pressure in the lower portions of the basin, if the point at which the trial is made is situate below the outcrop.

The primary rocks afford generally but little water. Having been subjected to violent convulsions, they are thrown into every possible position and broken by numerous fissures; and as no permeable stratum is interposed, as in the more recent formations, no reservoir of water exists. In the unstratified rocks, the water circulates in all directions through the fissures that traverse them, and thus occupies no fixed level. It is also impossible to discover by a surface examination where the fissures may be struck by a boring. For purposes of water supply, therefore, these rocks are of little importance. It must be remarked here, however, that large quantities of water are frequently met with in the magnesian limestone and the lower red sand, which form the upper portion of the primary series.

Joseph Prestwich, jun., in his ‘Geological Inquiry respecting the Water-bearing Strata round London,’ gives the following valuable epitome of the geological conditions affecting the value of water-bearing deposits; and although the illustrations are confined to the Tertiary deposits, the same mode of inquiry will apply with but little modification to any other formation.

The main points are—

The extent of the superficial area occupied by the water-bearing deposit.

The lithological character and thickness of the water-bearing deposit, and the extent of its underground range.

The position of the outcrop of the deposit, whether in valleys or hills, and whether its outcrop is denuded, or covered with any description of drift.

The general elevation of the country occupied by this outcrop above the levels of the district in which it is proposed to sink wells.

The quantity of rain which falls in the district under consideration, and whether, in addition, it receives any portion of the drainage from adjoining tracts, when the strata are impermeable.

The disturbances which may affect the water-bearing strata, and break their continuous character, as by this the subterranean flow of water would be impeded or prevented.